The broad objective is to understand the role and importance of uracil-DNA glycosidase in DNA repair. This enzyme removes uracil from both A-U and G-U basepairs; the former will result if dUTP is incorporated into DNA instead of dTTP, while the latter will be produced if cytosine in DNA is deaminated in situ. Both kinds of basepairs might affect the functioning of the DNA, but only cytosine deamination should be mutagenic. By use of the appropriate mutants, uracil-containing E. coli and viral DNA can be synthesized in vivo. DNA containing various amounts of uracil will be synthesized, and its biological functionality will be determined. The effects of the inability to excise uracil from DNA will be studied in various systems. The feasibility of using uracil-containing DNA for cloning recombinant DNA will be evaluated. The possible role of 5-methyl cytosine in mutagenesis in plants will be investigated. Preliminary experiments to study DNA repair in mouse brain will be initiated.